Process for isolating acrylic acid from the reaction gases obtained by the oxidation of propylene or a acrolein

ABSTRACT

Process for isolating acrylic acid from the reaction gases obtained by the oxidation of propylene or acrolein, wherein the hot reaction gases, which have a temperature of 300* to 600*C, are pre-cooled, wherein the resulting aqueous acrylic acid solution is heated to be freed from the acrolein contained therein, and wherein the resulting vapors, together with nonliquefied, acrolein-containing residual gas, are returned to the oxidation zone, the said process comprising the steps of precooling the reaction gas inside a heat exchanger down to a temperature of 100* to 200*C, subjecting thereafter the precooled gases to direct scrubbing with water having a temperature of 10* to 50*C and thereby cooling them further down to a temperature between 30* and 90*C, and heating the resulting aqueous, 10 to 45 percent acrylic acid solution to a temperature of about 100* to 120*C to expel residual acrolein therefrom.

United States Patent 1 Sennewald et al.

[54] PROCESS FOR ISOLATING ACRYLIC ACID FROM THE REACTION GASES OBTAINEDBY THE OXIDATION OF PROPYLENE OR A ACROLEIN [75] Inventors: KurtSennewald; Alfred Hauser; Klause Gehrmann, all of Knapsack near Cologne;Karl-Heinz Steil, Burbach near Cologne; Winfried Lork, Friesheim nearEuskirchen, all of Germany [73] Assignee: Knapsack Aktiengellschaft,Knapsack near Cologne, Germany [22] Filed: July 20, 1967 [21] Appl. No.2654,876

[] Foreign Application Priority Data 1 Feb. 20, 1973 FOREIGN PATENTS ORAPPLICATIONS Primary ExaminerLorraine A. Weinberger AssistantExaminer-Richard D. Kelly Attorney-Connolly and I-Iutz [5 7 ABSTRACTProcess for isolating acrylic acid from the reaction gases obtained bythe oxidation of propylene or acrolein, wherein the hot reaction gases,which have a temperature of 300 to 600C, are pre-cooled, wherein theresulting aqueous acrylic acid solution is heated to be freed from theacrolein contained therein, and wherein the resulting vapors, togetherwith nonliquefied, acrolein-containing residual gas, are returned to theoxidation zone, the said process comprising the steps of pre-cooling thereaction gas inside a heat exchanger down to a temperature of 100 to200C, subjecting thereafter the pre-cooled gases to direct scrubbingwith water having a temperature of 10 to C and thereby cooling themfurther down to .a temperature between 30 and C, and heating theresulting aqueous, 10 to 45 percent acrylic acid solution to atemperature of about to C to expel residual acrolein therefrom.

6 Claims, 2 Drawing Figures PROCESS FOR ISOLATING ACRYLIC ACID FROM THEREACTION GASES OBTAINED BY THE OXIDATION OF PROPYLENE OR A ACROLEIN Thecatalytic oxidation of propylene or acrolein in the gas phase is knownto be an important commercial process for the production of acrylicacid. In this process, propylene and/or acrolein are reacted with oxygenin the presence of oxidation catalysts (for example, oxides of theelements comprising molybdenum, chromium, vanadium, iron, cerium,titanium, nickel, tungsten, bismuth, tin, antimony, cobalt andphosphorus), at temperatures between 300and 600C,

to give acrylic acid. The reaction which is strongly exothermal, callsfor the presence of inert gases as a diluent. These include, forexample, the carbon oxides evolved by decomposition during the reactionor-in discontinuous processesthe nitrogen included in the air as theoxidizing agent. To increase the selectivity of this reaction, it isfurthermore necessary to provide for the presence of steam, which isadded to the starting gas in a proportion of to 60 percent by volume,The carbon monoxide and steam undergo conversion and partially producehydrogen and carbon dioxide. These latter gases are also inert diluents.The reaction results in the formation of a gas mixture with a content ofacrylic acid not higher than 0.2 up to a maximum of 3 percent by volume,depending on the reaction conditions. Unreacted propylene and/oracrolein are isolated from the acrylic acid and returned to the reactor.

The isolation of acrylic acid has been achieved heretofore by lowtemperature cooling of the reaction gases. For example, reference ismade to British Patent 948,687 and its disclosure not only of acrylicacid production by propylene-oxidation, which does not form part of thepresent invention, but also of acrylic acid-isolation from the reactiongases originating from the oxidation of propylene and acrolein, whereinthe reaction gases, that have a temperature of 280to 450C and are formedsubstantially of propylene, acrolein, acrylic acid, oxygen, inert gasesand steam, are indirectly cooled down to a temperature of about C,wherein the resulting aqueous acrylic acid solution is heated, forexample by injecting steam, to expel the acrolein contained therein, andwherein the resulting vapors, together with non-liquefiedacrolein-containing residual gas, are returned, after addition of freshpropylene, oxygen and steam, to the oxidation zone. As taught in BritishPatent 953,763, the low temperature cooling of the reaction gases canalso be achieved by scrubbing them directly with an acrylic acidsolution pre-cooled, for example to a temperature of 5to 10C, andthereafter cooling them indirectly in low temperature-heat exchangers.In either of these two processes, not only the acrylic acid, but alsothe total steam, comprised of the steam added as the diluent and thesteam evolved during the reaction, as well as the bulk of unreactedacrolein are subject to condensation; the acrolein is required to bedistilled off from the aqueous acrylic acid solution and to be added tothe starting gas, which in turn is required to be saturated again bymeans of steam.

Though considerable commercial and technical disadvantages comprisingthe erection of large refrigerating facilities and considerable loss inenergy, originating from the cooling and subsequent evaporation steps,have been found to be associated with the isolation of reaction with theresultant formation of pyranederivatives, i.e., so-called acroleindimers and trimers. At the same time, the homo and copolymerization ofacrolein and acrylic acid are found to be catalyzed by the elevatedtemperatures and the oxygen present in the gas.

The disadvantages reported above are obviated in the process of thepresent invention, which enables acrylic acid to be recovered almostquantitatively in technically simple manner and under economicconditions, without-any loss of acrolein, from the reaction gasesobtained by the oxidation of propylene and/or acrolein, by extraction ofthose gases with water at elevated temperature. The temperature at whichthe acrylic acid is extracted, is selected to obviate liquefaction ofthe acrolein and steam needed for the reaction. After the propylene oracrolein and the oxygen consumed in the reaction have been replaced withfresh gases, it is possible to return the reaction mixture to thereactor, without any further addition of steam. The acrylic acid isobtained in the form of an aqueous solution free of acrolein.

The process of the present invention will now be described withreference to FIGS. 1 and 2 of the accompanying drawing.

Hot reaction gas with a temperature of about 300to 600C leaving thereactor, is caused to travel through line 2 to heat exchanger 3 to becooled therein down to a temperature of l00to 200C, preferably ll0to150C. The heat set free during that operation is used for the generationof steam. The gas then flows through line 4 into column 5-usually a trayor packed column-in which the gas is cooleddown to a temperature lowerthan 100C, preferably 30up to C, by means of fresh water suppliedthrough line 6 near the column head, whereby acrylic acid is extractedfrom the reaction gas. The gas enters the column at a constanttemperature, but the fresh water-temperature, usually l0to 50C,preferably 20to 40C, is regulated in each particular case, depending onthe outlet-temperature desired for the gas, at the head of column 5. Thegas outlet-temperature in turn determines the steam partial pressuredesiredto prevail in the issuing gas. The quantity of fresh water neededis determined primarily by the concentration desired for the acrylicacid solution, usually 10 to 35 percent. In the case of rather prolongedoperation periods, it has been found advantageous to add to the freshwater small proportions of inhibitors for unsaturated compounds, e.g.,hydroquinone, methylene Blue, pyrogallol, p-tertiary butyl pyrocatechol,tertiary butyl catechol, thiodiphenyl amine, butyl mercaptane,thiglycolic acid, mercaptobenzthiazol, copper acetate, copper acrylateor copper oleate. The acrylic acid solution leaving column 5, is freedfrom traces of dissolved acrolein by heating the said solution to to Cin stripping column 8, that is connected either directly to scrubbingcolumn 5 (FlG. l) or connected to be parallel with that column 5 (FIG.2). As shown in FIG. 1, gaseous acrolein is introduced either directlyinto the ascending reaction gas stream, or is supplied through line 1 togas stream 7, as shown in FIG. 2. Acrylic acid solution free fromacrolein can be withdrawn from the bottom portion of column 8. Line 7 isthe discharge line for gas freed from acrylic acid, the gas containingall of the unreacted acrolein, inert gases and the desired proportion ofsteam. After the addition of fresh propylene or acrolein and oxygen, thegas is returned to the reactor.

The acrylic acid can be extracted at atmospheric pressure, or underslightly elevated or reduced pressure, preferably under a pressurebetween 0.5 and 3 atmospheres absolute.

The advantage associated with the isolation of acrylic acid by theprocess of the present invention is seen to reside in the energeticallywell-balanced conditions under which the process is carried out. Thecostly lowtemperature cooling and subsequent re-evaporation are replacedby an extraction step which requires no more than the use ofcommercially inexpensive water of circulation. The short sojourn time ofpolymerizable substances inside the column obviates theirpolymerization, even at elevated temperatures and ensures high yields ofmonomeric acrylic acid.

The present process for isolating acrylic acid from reaction gasesobtained by the oxidation of propylene or acrolein, wherein the hotreaction gases, which have a temperature of 300to 600C and are formedessentially of acrolein, acrylic acid, oxygen, inert gases, steam andpossibly propylene, are cooled, wherein the resulting aqueous acrylicacid solution is heated to expel the acrolein contained therein, andwherein the resulting vapors, together with non-liquefied,acroleincontaining residual gas, are returned to the oxidation zone,comprises more especially pre-cooling the hot reaction gases inside aheat exchanger down to a temperature of 100to 200C, preferably 1 10to150C, subjecting thereafter the said pre-cooled gases to directscrubbing with water having a temperature of l0to 50C, preferably to 40Cand thereby cooling them further down to a temperature between 30and90C, and heating the resulting aqueous, 10 to 45 percent acrylic acidsolution to a temperature of about 100to 120C to expel residual acroleintherefrom. The acrylic acid solution can be used in admixture with 0.001to 2 percent by weight of a customary stabilizer. The precooled reactiongases are preferably scrubbed with 50 to 500 cc water per normal cubicmeter (measured at N.T.P.) reaction gas.

EXAM PLE 1 1,037 normal liters/hour reaction gas having a temperature of410C, formed of propylene, oxygen and inert gases as well as of 1.5percent by. volume acrylic acid, 4.8 percent by volume acrolein and 23.2percent by volume (=24l normal liters) steam, were pre-cooled inside aheat exchanger, under a pressure of 1.2 atmospheres absolute, down to atemperature of 110C. Thev pre-cooled reaction gas was introducedthereafter into a tray column 80 cm wide with 24 trays, which was fed,per hour, with 160 cc water at 20C, supplied near the column head. A gastemperature of 64to 65C was found to prevail in the column. This enabled973 normal liters, per hour, of a gas mixture containing propylene,oxygen and inert gases as well as 5.1 percent by volume acrolein (=99percent of the acrolein used), 19.8 percent by volume (=193 normalliters) steam and very minor traces of acrylic acid, to be removed atthe column head and returned to the reactor. The 241 normal liters steamcontained in the initial reaction gas mixture, were comprised of 193normal liters cycled steam plus 48 normal liters reaction steam, whichwas required to be condensed during each passage and appeared in theacrylic acid solution.

The hot acrylic acid solution leaving the tray column, was introducedinto a packed column (30 X800 mm), the bottom portion of which wasmaintained at a temperature of 103to 105C to evaporate last traces ofacrolein. The acrylic acid solution was stabilized by the addition of0.1 percent by weight methylene Blue. In

this manner, there were obtained, per hour, 249 grams of an aqueous,19.7 percent by weight acrylic acid solution, corresponding to a yieldof 98 percent, referred to the acrylic acid contained in the reactiongas.

' EXAMPLE 2 2,600 normal liters/hour of a reaction gas mixture having atemperature of 450C, formed of propylene,

oxygen and inert gases as well as of 1.76 percent by volume acrylicacid, 6.92 percent by volume acrolein and 21.3 percent by volume (=554normal liters) steam, were pre-cooled down to a temperature of C,substantially in the manner set forth in Example 1, under a pressure of-1.1 atmospheres absolute, and introduced thereafter into the tray column(extraction column), which was fed, per hour, with 200 cc water at 20C,supplied near the column head. A gas temperature of 63to 65C was foundto prevail in the column. This enabled 2,467 normal liters of a gasmixture formed of propylene, oxygen and inert gases as well as of 7.3percent by volume acrolein, 18.95 percent by volume (=46? normal liters)steam and very minor traces of acrylic acid, to be removed at the columnhead and returned to the reactor. The 554 normal liters steam containedin the initial reaction gas mixture, were comprised of 467 normal literscycled steam plus 87 normal liters reaction steam, which was required tobe condensed during each passage and appeared in the acrylic acidsolution.

The hot acrylic acid solution leaving the tray column, was introducedinto the second column, the bottom portion of which was maintained at atemperature of 105to 106C to evaporate last traces of acrolein, which isintroduced into the cycled gas. The acrylic acid solution was stabilizedby the addition of 0.05 percent by weight methylene Blue. In thismanner, there were obtained, per hour, 411 grams of an aqueous, 34.4percent by weight acrylic acid solution, corresponding to a yield of96.2 percent, referred to the acrylic acid contained in the reactiongas.

The above examples shown that the content of steam in the gas cycledsubstantially does not depend on the water-content of the aqueousacrylic acid solution.

We claim: 7

1. 1n the process for isolating acrylic acid from the reaction gasesobtained by the oxidation of at least one member selected from the groupconsisting of propylene and acrolein, wherein the hot reaction gases,

which have a temperature of 300 to 600C and are formed essentially ofacrolein, acrylic acid, oxygen, inert gases and steam, are cooled,wherein the resulting aqueous acrylic acid solution is heated to expelthe acrolein contained therein, and wherein the resulting vapors,together with non-liquefied acrolein-containing residual gas, arereturned to the oxidation zone, the improvement which comprisespre-cooling the hot reaction gases inside a heat exchanger down to atemperature of 100 to 200C, subjecting thereafter the said pre-cooledgases in a tray or packed column to direct scrubbing with water having atemperature of to 50C and thereby cooling them further down to atemperature between 30 and 90C so as to provide a residual gas forreturn to the oxidation zone which includes steam in an amount of atleast 5 percent by volume of the gas subjected to oxidation, and heatingthe resulting aqueous, 10 to 45 percent acrylic acid solution to atemperature of about 100 to 120C to expel residual acrolein therefromand returning the resultant vapors together with the non-liquefiedresidual gas, which contains acrolein and the steam which is needed forthe reaction, to the oxidation zone.

2. The process of claim 1, wherein the said reaction gases also containpropylene.

3. The process of claim 1, wherein the said reaction gases arepre-cooled inside the said heat exchanger down to a temperature of 1 10to C.

4. The process of claim 1, wherein the scrubbing water has a temperatureof 20 to 40C.

5. The process of claim 1, wherein the said precooled reaction gases arescrubbed with 50 to 500 cc water per normal cubic meter reaction gas.

6. The process of claim 1, wherein the aqueous acrylic acid solution isstabilized by the addition of 0.001 to 2 percent by weight of acustomary stabilizer. I

1. In the process for isolating acrylic acid from the reaction gasesobtained by the oxidation of at least one member selected from the groupconsisting of propylene and acrolein, wherein the hot reaction gases,wHich have a temperature of 300* to 600*C and are formed essentially ofacrolein, acrylic acid, oxygen, inert gases and steam, are cooled,wherein the resulting aqueous acrylic acid solution is heated to expelthe acrolein contained therein, and wherein the resulting vapors,together with non-liquefied acrolein-containing residual gas, arereturned to the oxidation zone, the improvement which comprisespre-cooling the hot reaction gases inside a heat exchanger down to atemperature of 100* to 200*C, subjecting thereafter the said pre-cooledgases in a tray or packed column to direct scrubbing with water having atemperature of 10* to 50*C and thereby cooling them further down to atemperature between 30* and 90*C so as to provide a residual gas forreturn to the oxidation zone which includes steam in an amount of atleast 5 percent by volume of the gas subjected to oxidation, and heatingthe resulting aqueous, 10 to 45 percent acrylic acid solution to atemperature of about 100* to 120*C to expel residual acrolein therefromand returning the resultant vapors together with the non-liquefiedresidual gas, which contains acrolein and the steam which is needed forthe reaction, to the oxidation zone.
 2. The process of claim 1, whereinthe said reaction gases also contain propylene.
 3. The process of claim1, wherein the said reaction gases are pre-cooled inside the said heatexchanger down to a temperature of 110* to 150*C.
 4. The process ofclaim 1, wherein the scrubbing water has a temperature of 20* to 40*C.5. The process of claim 1, wherein the said pre-cooled reaction gasesare scrubbed with 50 to 500 cc water per normal cubic meter reactiongas.